The present invention relates to a method of initiating regeneration of a particulate filter for a direct-injection diesel engine with a common rail injection system.
As is known, in many countries, the regulations governing atmospheric pollution are becoming increasingly strict with regard to the composition of internal combustion engine exhaust gas.
In the case of diesel engines in particular, the main problems are posed not so much by carbon monoxide (CO) and hydrocarbons (HC) as by nitric oxide (NOx) and particulate in the exhaust gas.
Numerous methods have been proposed whereby to minimize the particulate content of exhaust gas emitted into the atmosphere. Of these, fitting the exhaust pipe with a particulate filter has long been acknowledged in engine technology as undoubtedly the final solution to the problem of diesel engine particulate emissions.
A diesel particulate filter (DPF)xe2x80x94also known as a particulate trap (soot catcher or bare trap)xe2x80x94normally comprises a number of parallel channels with alternating porous barrier walls.
More specifically, the barriers force the exhaust gas to flow through the lateral walls of the channels, so that the unburned particles constituting the particulate are first retained in the pores of the lateral walls, and, when the pores eventually become clogged, accumulate and form a porous layer on the inner surfaces of the channel walls.
As the particulate accumulates on the inner surfaces of the channel walls, the pressure drop through the filter, and therefore the backpressure generated by the filter, also increases.
If not removed eventually, an excessive accumulation of particulate therefore results in:
impaired performance, driving comfort, and consumption of the engine, until the engine eventually stalls; and
destruction of the filter itself in the event of self-firing and uncontrolled combustion of the particulate. That is, in particular driving conditions, a large accumulation of particulate may give rise to xe2x80x9ccriticalxe2x80x9d regeneration phenomena, in turn resulting in sudden, uncontrolled particulate combustion, overheating of the ceramic matrix of the filter, and therefore damage to the filter itself.
The trapped particulate must therefore be removed regularly by xe2x80x9cregeneratingxe2x80x9d the particulate filter, which, in engine technology, means burning the accumulated particulate (substantially carbon, C) which, in contact with the oxygen in the exhaust gas, is converted into CO and CO2.
This reaction, however, only occurs naturally (i.e. without using additives) at temperatures over roughly 600xc2x0 C., which are much higher than those at the filter inlet in normal engine operating conditions.
In certain conditions, i.e. on detecting a given accumulation of particulate in the filter, the exhaust gas temperature at the filter inlet must therefore be increased artificially to 600xc2x0 C. to initiate particulate combustion.
Regeneration of particulate filters is the main problem posed by use of this sort of filter in the automotive industry.
Numerous methods of artificially increasing exhaust gas temperature at the filter inlet to initiate particulate combustion have been proposed and actually implemented.
These roughly fall into two major categories, depending on the approach adopted: a first category based on the use of a fuel additive, which acts as a catalyst to reduce regeneration initiation temperature by roughly 100-150xc2x0 C.; and a second category involving no fuel additive.
Additive-based methods of initiating particulate combustion require:
an exhaust system comprising a catalyst and particulate filter integrated in a single canister;
a high-volume particulate filter, typically equal to twice the engine displacement;
a fuel additive (cerium-based) to reduce regeneration initiation temperature by 100-150xc2x0 C.;
a highly complex on-vehicle automatic additive feed and metering system; and
engine control strategies to increase temperature at the filter inlet, on account of the required temperatures not being achievable in normal engine operating conditions. This type of system in fact only operates correctly in medium-load engine operating conditions; whereas, in the case of prolonged low-load operation (e.g. in city traffic) and/or low external temperatures (in winter), the exhaust gas often fails to reach initiation temperature.
Additive-based particulate combustion initiation methods provide for initiating regeneration of particulate filters at about 450-500xc2x0 C. with a low backpressure generated by the filter, but have significant drawbacks which prevent them from being used to full advantage:
they are complex, particularly as regards the automatic additive feed and metering system;
a high-volume particulate filter must be installed, on account of the additive in the fuel leaving a gradually increasing ash deposit in the filter;
despite the high volume of the particulate filter, the ash must still be xe2x80x9ccleaned offxe2x80x9d roughly every 80,000 km; cerium, in fact, produces large amounts of ash which accumulate, together with the particulate, inside the filter, and which cannot be eliminated by regeneration; as a result, the backpressure of the filter gradually increases with mileage, so that the filter must be dismantled and cleaned regularly to remove the accumulated ash;
high cost of both the automatic additive feed and metering system and the high-volume particulate filter.
On account of the above drawbacks, non-additive-based particulate combustion initiation methods are now preferred by most car manufacturers.
One solution proposed and implemented to increase the exhaust gas temperature in particulate filters artificially without recourse to an additive comprises fitting the particulate filter with heating elements which are activated periodically to heat the filter to the temperature initiating combustion of the trapped particulate.
More recently, however, solutions have been proposed whereby the temperature of the exhaust gas at the particulate filter inlet is increased by engine control strategies.
The most commonly used strategies to increase temperature at the particulate filter inlet comprise:
acting on the main injection to delay combustion;
post-injection; or
reducing air intake (e.g. by reducing supercharging or throttling intake).
The delayed main injection based strategy is limited by the main injection only being delayable up to a certain point, beyond which combustion would become unstable, thus resulting in misfiring, white/blue smoke, and driving problems, in particular xe2x80x9cswooningxe2x80x9d phenomena. In low engine speed and load conditions, in particular, this strategy therefore fails to achieve high temperatures at the filter inlet.
In European Patent Application WO 96/03571 filed by the present Applicant, on the other hand, a strategy is proposed whereby the temperature of the exhaust gas at the particulate filter inlet is increased by performing, in addition to the main injection, a post-injection at the expansion stroke.
Timing of the post-injection with respect to the main injection and the amount of fuel injected are so determined that fuel combustion at the expansion stroke increases the temperature of the exhaust gas sufficiently to initiate regeneration of the particulate filter.
In European Patent Application WO 96/03572, also filed by the present Applicant, a strategy is proposed whereby the temperature of the exhaust gas at the particulate filter inlet is increased by performing, in addition to the main injection, a post-injection at the exhaust stroke.
More specifically, since the particulate filter is normally integrated in a single canister together with a DeNOx catalyst upstream from the particulate filter, a post-injection performed predominantly at the exhaust stroke means the injected fuel has no, or only very little, effect on combustion and is therefore supplied, unburned, directly to the catalyst.
The unburned hydrocarbons thus introduced into the catalyst initiate an exothermic oxidation reaction which increases the temperature of the exhaust gas at the catalyst outlet and therefore at the particulate filter inlet.
Unlike additive-based methods, non-additive-based particulate combustion initiation methods require:
a particulate filter of a volume substantially equal to the engine displacement, i.e. half the volume required by additive-based particulate combustion initiation methods;
an exhaust system which may either:
have the same configuration as that required by additive-based particulate combustion initiation methods, i.e. comprising a catalyst and particulate filter integrated in a single canister; or
comprise one filter on which are deposited both the oxidizing elements of the catalyst, and initiation-temperature-reducing metals (Ce+Pt) (catalyzed soot filter);
no fuel additive;
no on-vehicle automatic additive feed and metering system;
engine control strategies to increase the temperature at the particulate filter inlet.
Using an exhaust system comprising a catalyst and particulate filter integrated in a single canister, regeneration is initiated t around 600xc2x0 C., and the particulate filter generates a low backpressure; whereas, using an exhaust system comprising one filter on which are deposited both the oxidizing elements of the catalyst and initiation-temperature-reducing metals, regeneration is initiated at around 450xc2x0 C., but the particulate filter generates a high backpressure, and regeneration efficiency may be impaired by cesium-particulate contact and may also drift, i.e. resulting in an increase in regeneration initiation temperature.
It is an object of the present invention to provide an improved method of initiating regeneration of a particulate filter, designed to eliminate the drawbacks of the above methods, i.e. which requires no fuel additive; guarantees exhaust gas temperatures of over 600xc2x0 C. at the particulate filter inlet, thus initiating regeneration of the particulate filter, in all engine operating conditions; and provides for stable combustion during regeneration to ensure good driving conditions with no xe2x80x9cswooningxe2x80x9d phenomena and no white/blue smoke at the exhaust caused by misfiring.
According to the present invention, there is provided a method of initiating regeneration of a particulate filter for a diesel engine with a common rail injection system, as claimed in claim 1.